Because the utilization of the Internet is raised more and more in families and enterprises, the situation of sharing resources by a local area network (LAN) constituted by plural personal computers 11 through a switch 10 becomes more and more popular. For reducing the trouble occurred upon the wiring construction in building up a network, the wireless network device seems to be a better way to solve this problem (please refer to FIG. I showing the relevant schematical view). Among these, the 802.11 is a standard appointed by the IEEE in 1997 and used in common for the wireless network. For solving the problem about the signals being intercepted to then cause the plaintext to be tapped when being transmitted wirelessly, the 802.11 appoints the wired equivalent privacy (WEEP) as the mode for encryption. Please refer to FIG. 2 which is a block diagram showing the function of the WEP encryption mode. The system manager has to appoint a set of secret keys having the length of 40 bits in advance, and then when some subscriber end wants to transmit a plaintext through the wireless network, the system manager will read the preset secret key, add a random produced initialization vector (IV) having a length of 24 bits to constitute a 64 bits WEP seed, and then execute WEP encryption algorithm to produce a key sequence. Additionally, after the plaintext is executed by an integrity check algorithm in an integrity check operator 21, an integrity check value (ICV) is produced to be attached to the plaintext. Thus, when the transmission end wants to process the plaintext transmission, an exclusive OR (XOR) operator 22 will execute an XOR operation for the key sequence and the plaintext containing the ICV to produce a ciphertext. As to the random produced IV, it is attached to the ciphertext without encryption, and the IV and the ciphertext are transmitted altogether to the receiving end. After receiving the signal, the receiving end operates the prestored known 40 bits secret key with the received IV to produce the same key sequence, so as to decrypt the ciphertext for obtaining the plaintext.
In the method described above, because all the IV, WEP encryption algorithm, and the XOR operation are public informations, the variation of the IV is limited, and the executive program of high level network protocol is fixed, the secret key can be easily decrypted by the eavesdropper under sufficient time and database. For avoiding the decryption of the secret key, the system manager has to change the user secret keys all the time. Consequently, the burden of the system manager is cumulated more and more, and actually this ideal is also hard to be achieved. Thus, it is therefore tried by the present application to deal with this situation.